The present invention relates to a special effect system for a television signal and, more particularly, to an apparatus for eliminating a noise component from a key signal used in the special effect system.
In a television special effect system, a plurality of video signals are combined together using a key signal, such as a chroma-key signal. In a special effect system employing a chroma-key signal, an input video signal is variously compressed in accordance with the width of an input key signal, and the compressed input video signal is combined with another video signal. When a noise component is included in the key signal, however, not only does the combined picture frame become distorted, but also the input video signal is not properly compressed, and the desired special effect is not realized.
A conventional approach to remove a noise component in a key signal is to remove a pulse component in the key signal having a pulse width narrower than a predetermined width. One such noise eliminating technique is disclosed in U.S. Pat. No. 4,628,363, entitled "Chroma-key Tracking Apparatus". In this technique, the pulse width of a pulse component of a key signal is measured by counting clock pulses during such pulse width, and when the counted value is less than a predetermined value, the pulse component is treated as noise and removed by closing a gate. The disclosed technique suffers, however, by narrowing the width of the output key signal occurring just after the noise removal. This necessitates processing to enlarge the affected output key signal.